1. Field of the Invention
The present invention relates to a manufacturing method of a heat-resistant alloy having excellent hot sulfidation-corrosion resistance suitable for use in apparatuses used in high temperature corrosion environments, particularly in sulfur-corrosion environment containing H2S, SO2, etc., such as expander turbines utilizing the energy recovered from exhaust gas from fluid catalytic cracking unit in a petroleum refining system, for example.
2. Description of the Related Art
Heat-resistant nickel-based alloys having excellent strength and corrosion resistance at elevated temperature have heretofore been widely used for members exposed to high temperatures, such as expander turbine rotors. A typical example of such alloys is what is known as Waspaloy (a registered trademark of United Technologies).
Heat-resistant nickel-based alloys used for members exposed to elevated temperatures usually gain their high temperature strength through the precipitation strengthening of intermetallic compounds called the xcex3xe2x80x2 phase. Since the xcex3xe2x80x2 phase has Ni3(Al, Ti) as its basic composition, Al and Ti are normally added to these alloys.
In high-temperature equipment exposed to a combustion-gas atmosphere, such as boilers, on the other hand, the so-called xe2x80x9chot corrosionxe2x80x9d phenomenon involving molten salts such as sulfates, V, Cl, etc., is known. It is reported that sulfidation corrosion caused by the direct reactions of gases not involving molten salts with metals occurs with nickel-based alloys at approximately 700xc2x0 C. or higher. This phenomenon is attributable to the formation of a liquid phase of Nixe2x80x94Ni3S2 eutectics.
In order to accomplish energy conservation in oil refineries, on the other hand, a system for recovering energy in the exhaust gas generated from the fluid catalytic cracking unit has been developed. When Waspaloy, a typical Ni-based superalloy, was used for gas-expander turbine blades in such equipment, sulfur corrosion occurred at the roots of the rotor blades though it was used in a temperature region far lower than the temperature heretofore considered critical.
Closer scrutiny of this phenomenon revealed that although corrosion developed along grain boundaries, no molten salts were present at corroded areas, indicating that the corrosion was caused by the direct reactions of the metal with gases. Such an intergranular sulfidation corrosion of a Ni-based superalloy in a sulfur-laden gas environment containing no molten salts in a temperature region lower than the eutectic point of Nixe2x80x94Ni3S2 has been scarcely observed in the past.
To solve this problem, the inventors of U.S. Pat. No. 5,900,078 issued May 4, 1999 studied in detail the effects of alloy elements on the sulfidation behavior of Waspaloy in a sulfur-laden gas environment in a temperature region lower than the eutectic point of Nixe2x80x94Ni3S2, and elucidated that the sulfidation layer in the alloy including grain boundaries is enriched in Ti, Al and Mo contained in the alloy, and that the Ti and Al contents of the alloy have a marked effect on the sulfidation-corrosion resistance of the alloy.
As a result, a hot sulfidation-corrosion-resistant Ni-based alloy containing 12 to 15% Co, 18 to 21% Cr, 3.5 to 5% Mo, 0.02 to 0.1% C, not more than 2.75% Ti and not less than 1.6% Al, with the balance substantially comprising Ni, excluding impurities, has been proposed, as disclosed in U.S. Pat. No. 5,900,078.
The alloy disclosed in U.S. Pat. No. 5,900,078 has attracted trade attention as a heat-resistant Ni alloy whose hot sulfidation-corrosion resistance has been dramatically improved by reducing the Ti content and increasing the Al content among the known addition elements of Waspaloy.
The present inventor et al., however, made clear after further study of the alloy that the sulfidation-corrosion resistance, particularly corrosion resistance at the alloy grain boundaries, that is, intergranular sulfidation-corrosion resistance of even the alloy having improved hot sulfidation-corrosion resistance, as disclosed in U.S. Pat. No. 5,900,078 could be changed if manufactured with difference methods. The same hold true with Waspaloy that has been widely known.
Since heat treatment conditions for these heat-resistant Ni alloys have often been determined, placing emphasis mainly upon strength characteristics and hot workability, the resulting alloys have not necessarily shown good hot sulfidation-corrosion resistance.
It is therefore an object of the present invention to provide a manufacturing method, particularly a heat treatment method for improving the sulfidation-corrosion resistance of the sulfidation-corrosion-resistant Ni-based alloy disclosed in U.S. Pat. No. 5,900,078 and other Ni-based alloys used for members of corrosion-resistant high-temperature equipment while maintaining the same high-temperature strength characteristics as those of conventional alloys.
After studying the intergranular sulfidation-corrosion characteristics of the hot sulfidation-corrosion resistant Ni-based alloy disclosed in U.S. Pat. No. 5,900,078 and Waspaloy, which were subjected to various heat treatment processes, the present inventor et al. discovered that grain boundaries are corroded because carbides chiefly consisting of Cr are precipitated in the grain boundaries, causing Cr to reduce in the vicinity of grain boundaries, and Cr-depleted zones to be formed along the grain boundaries. Consequently, the present inventor et al. have conceived the present invention based on the assumption that sulfidation corrosion at grain boundaries can be controlled by inhibiting the formation of Cr-depleted zones at the grain boundaries.
That is, the present invention is a manufacturing method of a Ni-based alloy containing 0.005 to 0.1% C, 18 to 21% Cr, 12 to 15% Co, 3.5 to 5.0% Mo, not more than 3.25% Ti, and 1.2 to 4.0% Al in mass percent, with the balance substantially consisting of Ni, and a manufacturing method of a Ni-based alloy having improved sulfidation-corrosion resistance which is, after solid solution heat treatment, subjected to stabilizing treatment for 1 to 16 hours at not lower than 860xc2x0 C. and not higher than 920xc2x0 C., and aging treatment for 4 to 48 hours at not lower than 680xc2x0 C. and not higher than 760xc2x0 C.
More preferably, the present invention is a manufacturing method of a Ni-based alloy having improved sulfidation-corrosion resistance which is subjected to secondary aging treatment for not less than 8 hours at not lower than 620xc2x0 C. and not higher than an aging treatment temperature minus 20xc2x0 C.
The present invention is a manufacturing method of a Ni-based alloy having improved sulfidation-corrosion resistance whose desirable alloy composition is Ti: not more than 2.75%, Al: 1.6 to 4.0% in mass percent, and more preferably any one type of B: not more than 0.01%, or Zr: not more than 0.1% in mass percent.